1. Field of the Invention
This invention relates generally to the creation of ions within an ion guide. More specifically, the present invention superimposes a symmetrical magnetic field around an ion guide to thereby prolong interaction between electrons, uncharged compounds, charged compounds, resulting in enhanced ion creation.
2. Description of Related Art
The relevant state of the art in the production of gas phase ions requires examination of both electron ionization and chemical ionization. In general, ionization is used for many purposes, including providing a continuous stream of ions, trapping and confining ions for pulsed production, and functioning as a source of first ions that generate secondary ions. Gas phase ions are often created in order to perform some type of analysis, such as in an ion mobility analyzer, a mass analyzer, and a secondary ion mass spectrometer.
It is useful to look at a specific example of how ionization is performed and used in order to understand the advantages of the present invention. Accordingly, the deficiencies inherent in ionization sources when used with a mass analyzer will be explained.
The principle of mass spectrometry (MS) is the production of gas phase ions that are subsequently separated and detected according to their m/z values. A mass spectrometer consists of five basic parts: sample introduction system, ionization source, mass analyzer, ion detector, and a data acquisition/handling system. While there are many different techniques to accomplish ionization, the most widely used technique is electron ionization (EI) due to its high sensitivity, ease of generating high electron emission current, approximately uniform ionization yield for most analytes; rich structural information in resulting mass spectra, and extensive 70 eV EI mass spectral libraries that enable spectral matching and compound identification.
EI sources have been extensively investigated and various designs have been published and converted into commercial products. Most EI sources are based on the design of Nier. Electrons from an electrically heated filament (cathode) are accelerated by a potential difference between the filament and the source enclosure, and pass through the entrance and exit apertures in the source body. This electron beam penetrates a sample vapor at reduced pressure in the source volume. In most cases, the approach of an energetic electron near an analyte molecule leaves the molecule with a positive charge as the exchange of energy results in the ejection of an electron from the molecule. The dimensions of the electron beam determine the ionization volume in which ions can be formed. Ionization takes place only in the area of the gas flow that is traversed by the electron beam, so that the number of ions formed is relatively small. Moreover, only some of these ions can then be channeled by additional focusing into the mass analyzer.
Because ion source efficiency is the main factor limiting sensitivity in many types of mass spectrometers, many applications of MS are determined in large part by this efficiency; therefore, it is important to study means for improvement of ionization efficiency.
One technique to improve ionization efficiency is to increase the sample pressure and thereby increase the density of molecules in the ionization region. Higher pressure in a small source volume will provide more effective use of the sample. Another technique is to increase the electron emission by heating the filament to a higher temperature, thus, increasing the number of electrons entering the ion source. However, both methods have the disadvantage of decreasing filament lifetime. The former may also require isolation and increased pumping of the mass analyzer region to avoid degradation of analyzer performance due to ion/molecule interaction. The latter process cannot be continued indefinitely because the cloud of electrons formed above the hot filament surface reduces the emission efficiency due to space charge effects.
Ionization can also be enhanced in a number of other ways. One approach is to utilize the ionizing electrons as efficiently as possible. The electron beam can be focused into the ionization volume by the use of an electron repeller positioned behind the filament that is electrically connected to a more negative potential than the filament bias voltage. The electrons can also be reused by reflecting them back and forth many times in the potential well established between the negatively biased cathode and anode, allowing the electrons a greater opportunity to interact with sample molecules. A weak collimating magnetic field can be applied parallel to the electron beam axis, and the field strength can be chosen to provide high transmission of electrons with minimal perturbation of the ion beam. A field of 100 gauss is typically used. The magnetic field can cause electrons to follow a helical trajectory around the magnetic field lines, thereby increasing the effective electron ionization path length L and ionization probability.
Simulation of ion trajectories has led to improved source geometry and optimum electrode potentials to produce a focusing electric field inside the source volume, which effectively allows sampling from a larger ionization volume and leads to higher ion extraction efficiency. Usually, the ion source contains a series of ion optics to draw out, focus, and inject ions into the mass analyzer.
Recently, EI sources have been designed with large dimensions, especially for use with atomic or molecular beams. Electrons can be focused by using either a magnetic or an electric field. One prior art described an EI source that had a separate electron-generating chamber in which electrons were generated completely around, directed toward, and deflected longitudinally along the instrument axis and into the ionization chamber. The electron trajectories overlapped with the profile of gas flow along the source axis. This resulted in a much longer contact between the electrons and the gas. The generated ions were forced into a cylindrical beam of relatively small diameter.
Similarly, another reference teaches a cylindrically symmetric magnetic field that compressed electrons emitted from a large filament into a long narrow volume containing primarily the molecule beam. This has the advantage of both producing little background from residual gas and increasing the interaction region between the ionizing electron beam and the molecular beam. Experiments using thermal helium atoms showed very high efficiency and sensitivity.
Another interesting approach is internal ionization using an electron beam in a Paul ion trap. The electron beam is introduced inside the ion trap to ionize the sample molecules. The instrument sensitivity resulting from internal ionization can be high because all of the ions in a solid angle can be confined and detected.
Recently, an ion source consisting of four electrodes which create an approximate radio frequency (RF) quadrupole field in the center of the ionization volume was designed to enhance ion source performance for a quadrupole mass spectrometer. Background ions of low m/z, such as helium ions, could be ejected immediately after formation. Meanwhile, sample ions were focused toward the z-axis by the RF electric field by collisional cooling while moving toward the exit. A ten-fold increase in signal-to-noise (S/N) ratio (compared to a traditional ion source) was claimed by the authors. It is noted that RF should not be considered to be limited to those frequencies typically associated with radio waves. Those skilled in the art understand that RF includes a much broader range of frequencies.
The renaissance of time-of-flight mass spectrometry (TOFMS) can be primarily attributed to the development of two new ionization methods, matrix-assisted laser desorption ionization and electrospray ionization. TOFMS has several features that are particularly useful for analysis of large bio-molecules. First, the m/z range is theoretically unlimited. Second, no defining slits are needed and ions can be detected over the complete m/z range at the same time without scanning. Early TOFMS instruments suffered from poor resolution; however, electrostatic reflectors, orthogonal acceleration, and the rediscovery of the benefits of delayed extraction have led to substantial improvements in resolution. Consequently, TOFMS instruments now provide, in most cases, optimum combination of resolution, sensitivity, and speed, particularly under conditions in which the entire mass spectrum is required.
In contrast to pulsed ionization sources, there are difficulties in coupling any continuous source to a TOFMS instrument, including the EI source. A TOFMS instrument must be pulsed in some way, as there needs to be a reference (or well-defined start time) in order to deduce a flight time from the detected ion arrival time. Thus, the main challenge in coupling continuous ion sources is producing temporary discrete ion packets, either by pulsing the source or gating the ion beam. This imposes a serious limitation in ion sampling efficiency or duty cycle (i.e., ratio of ions detected to ions formed). Sometimes, exaggerated claims are made with regard to sensitivity of TOFMS instruments compared to scanning mass spectrometers when more than one mass is observed. Few approaches for coupling EI sources to TOFMS have yielded sufficient resolving power, sensitivity, and fidelity of ion abundances when compared to mass spectral libraries.
Combining an ion trap with orthogonal acceleration TOFMS has so far proven to be an effective method to greatly improve the duty cycle. An in-line ion-trap storage/TOFMS instrument in which ions could be either externally injected into (or internally formed in) a quadrupole ion trap has been shown in the prior art. This approach provided the potential for nearly 100% duty cycle in converting a continuous ion beam into a pulsed source for TOFMS. However, a cooling step was required, ions with a wide energy spread were ejected, it had limited charge capacity and trapping efficiency, and there was difficulty in ejection of ions at a high repetition rate.
Quadrupole, cylindrical, and segmented ring cylindrical ion traps have all been interfaced to TOFMS. In one study an ion trap El source was designed in which two thin diaphragms made up a segmented ring electrode and the end cap electrodes were constructed of planar wire mesh. The potential field produced by the RF voltage resembled that of a cylindrical ion trap. The linear extraction field enabled good resolution without the need for ion cooling prior to mass analysis. However, it was important to find the best RF phase to initiate extraction. Furthermore, with EI in the ion trap, the filament bias voltage was not a direct measure of the ionizing electron energy because of the large effect of the RF field on the electron trajectories. Nevertheless, an ion trap/TOFMS using an EI source was developed for real time monitoring of trace volatile and semi-volatile compounds in air. An ion storage time of 400 ms limited the maximum spectral acquisition rate and made it unsuitable for fast on-line separations. The possibility of coupling an ion trap and orthogonal acceleration TOFMS was also studied. Because of the wide ranges in ion velocities and ejection times, the triggering between the ion trap and orthogonal acceleration TOFMS was impossible for full mass spectral acquisition.
Orthogonal acceleration provides a high efficiency interface for converting ions from a continuous beam into segmented ion packets. This instrument offers high sensitivity at the high mass end of the spectrum. It should be noted that most of the ion sources used in prior art references were originally designed for quadrupole or magnetic sector instruments. Because they were not optimized for orthogonal acceleration TOFMS, they provided relatively low ion transmission efficiency and necessitated the use of complex ion optics to manipulate the ion beam.
Accordingly, what is needed is an ionization source that provides improved ionization efficiency from efficient use of electrons and a nontraditionally large ionization volume. This system should function as an ion source for many different applications, and should work with both electron and chemical ionization in a continuous and pulsed mode of operation.